(a) Field of the Invention
The present invention relates to a liquid electrolyte which can be used for carrying out chemical reductions. More particularly, the invention is concerned with a methanesulfonic catholyte containing a tetravalent salt of titanium which is dissolved in a methanesulfonic acid solution. The invention is also directed to an electrochemical process including the catholyte of the invention, an electrolyte process for the reduction of tetravalent titanium into trivalent titanium as well as the simultaneous oxidation of the reduced form of redox couple, when used as a reductant, for organic molecules such as nitrobenzenes, sulfoxides or quinones. Finally, the invention pertains to improved methods for the reduction of chemical compounds using a solution of trivalent titanium in methanesulfonic acid.
(b) Description of the Prior Art
Redox reagents are compounds that can exist in an oxidized or reduced state. Usually, these compounds are transition metals such as iron, chromium, manganese, vanadium, etc. Great use has been made of these compounds in organic synthesis for the oxidation or reduction of reactive groups. Examples include the oxidation of methyl groups to aldehydes or acids, introduction of quinone groups to aromatic ring systems, the reduction of nitro groups to amines and the addition of hydrogen to unsaturated molecules.
The manufacture of anthraquinone from the chromic acid oxidation of anthracene, with Subsequent re-oxidation of the chromic acid in an electrochemical cell is such an example. Such processes were used in the dyestuff industry in Germany as early as the turn of the century. Other processes involved the use of regenerated chromic acid to bleach montan wax, and the use of chromic acid to manufacture saccharine. Examples of using regenerated redox reagents abound in the literature of electrochemical synthesis. In some cases the redox reagents were added along with the organic substrate and the whole treated in an electrochemical cell. This is known as in-cell reaction. In other cases, the reagent was prepared electrochemically in solution, mixed with the organic substrate in a separate treatment, to so-called ex-cell method. This application concerns this latter approach.
Regardless of which system of redox manipulation is involved, the role of the redox reagent is to react easily and efficiently with the electrode on the one hand and the normally insoluble organic substate on the other. The role of the redox is then in the case of oxidation that of the oxygen to the organic substrate and is itself reduced. The reduced redox form being both soluble and able to contact the anode in the cell without hindrance is then easily re-oxidised ready for a further reaction with the organic substrate. In this way, redox reagents are used to enhance reaction rate between a poorly soluble reagent and the electrochemical transfer of electrons which accompanies oxidation or reduction of all chemical compounds. Electron transfer occurs essentially at a two-dimensional surface; consequently, at the electrode, poor soluble reactions have statistically a much shorter period in the vicinity of this surface for such electron transfers to take place in the poorly soluble substrate.
Ideally, redox reagents are chosen for their ability to bridge the solubility gap between the reagent to be oxidized or reduced and the regenerating electrode. A further property of redox reagents is to be considered in the selection for a particular process, namely the redox potential. This may be considered as a measure of their ability on a thermodynamic scale to oxidize or reduce other materials.
In the past, metals such as iron or zinc have been used in the presence of acids such as hydrochloric in the Bechamp process to reduce nitrobenzenes such as p-nitrotoluene and p-xylidene to their respective amines. However, these processes produce large quantities of contaminated metal oxides which require disposal and are therefore technologies which are damaging to the environment. Another process which can be used to reduce nitrobenzenes and other nitrated compounds is the catalytic reduction with hydrogen. This technology, however, is capital-intensive and therefore dedicated equipment can only be justified for such a process when there is a large demand for the product.
The efficiency of the direct electrolytic reduction is inhibited by the low solubility of the nitrobenzenes in the aqueous electrolyte and the poor conductivity of non-aqueous electrolytes in which these compounds are soluble.
The electrolytic reduction of titanium(IV) to titanium(III) is well known in acid media, such as hydrochloric, sulphuric, etc. as is disclosed in prior art by Udupa and others.
In sulphate media, the faradaic efficiency is poor because of low solubility and in chloride media, it is difficult to find electrode material which can withstand the corrosive nature of the solution. On the other hand, it is well known to reduce nitrobenzenes at the cathode of electrolytic cells. The introduction of titanium(IV) to the electrolyte improves the faradaic efficiency of the reduction of nitrobenzenes. However, the low solubility of titanium(IV) in sulphate does not allow the ex-cell use of the then-reduced titanium(III).
The simultaneous generation of chromium and other redox agents [titanium] has been disclosed by Chaussard et al in Canadian Patent 1,191,811. However, chromium(VI) is a powerful oxidising agent, and due to its oxygen donating ability, it is less selective than cerium(IV) which is a more powerful oxidising agent. Thus, chromium(VI) is a preferred oxidising agent for producing organic acids such as benzoic acid. Unfortunately, however, these acids are, for the most part, produced more economically by direct oxidation with oxygen under catholytic conditions.